DESCRIPTION

The present invention relates to a modular atmospheric burner.

The invention relates in particular to an atmospheric burner partially pre- mixed with gaseous fuel.

In atmospheric burners the gaseous fuel is mixed with a determined quantity of combustion air, so-called primary air, before combustion. The mixing between fuel and primary combustion air, comprised of atmospheric air, is obtained by injecting the fuel into the opening of a Venturi tube. The energy contained in the flow of fuel determines the drawing of primary air which is mixed with the fuel inside the Venturi tube itself. At the outlet of the Venturi tube, a mixing chamber is arranged, inside which the mixing between the fuel and the primary combustion air is completed. The mixing chamber substantially comprises an appropriately delimited volume which, at a first end, is connected to the outlet of the Venturi tube. At a second end of the mixing chamber a diffusor is arranged at which, outside the mixing chamber, the combustion develops and is completed thanks to a further supply of combustion air, known as secondary air, comprising ambient air that can be found outside the burner. The diffusor is fundamentally comprised of a relatively thin body through which the mixture of primary air and fuel flows towards the outside. This body comprises pierced or porous metal or ceramic material.

A fundamental objective that presides over the correct sizing of the burner is to reduce as much as possible the production of pollutant gases such as carbon monoxide (CO) and nitrogen oxides (NO and NO ₂ ). During combustion the nitrogen oxides are formed by the oxidation of the nitrogen contained in the combustion air. The formation of the nitrogen oxides takes place mainly in the areas of the flame where high temperatures and a high quantity of oxygen are present simultaneously.

In atmospheric burners, in order to reduce the production of nitrogen oxides the Venturi tube is usually structured so that the flow rate of primary air drawn and the flow rate of fuel injected are in a determined ratio and are appropriately mixed with each other. In cases in which the ratio between primary air and fuel gas is lower than the stoichiometric ratio, the amount of air missing in order to reach the stoichiometric value is represented by the secondary air that is taken by the flame outside the burner. The inflow of secondary air, as well as completing the combustion, cools down the temperature of the flame, contributing to reducing the production of nitrogen oxides. On the other hand, an excessive lowering of the temperature of the flame may produce an increase in the production of carbon monoxide (CO).

It is therefore fundamentally important to obtain the correct balance between the flow rate of primary air and the flow rate of secondary air in order to fulfil the best operating conditions of the burner.

Various aspects of atmospheric burners with low NOx currently available can be improved.

In particular, a good compromise has not been reached between the emissions of pollutants and the dimensions of the burners. In fact, their volume is still relatively high and does not permit functional assembly in devices such as current domestic atmospheric boilers. The surface area of the burning surface with respect to the nominal power of the boiler is also still very high in current domestic atmospheric boilers. Furthermore, traditional atmospheric burners with low NOx do not allow the modulation of power in continuous mode.

The object of the present invention is to offer a modular atmospheric burner which, whilst guaranteeing the same levels of CO emissions, allows the drawbacks of the burners currently available to be overcome.

An advantage of the burner according to the present invention is that it allows the production of nitrogen oxides to be remarkably reduced with respect to the burners currently available for similar uses.

Another advantage of the burner according to the present invention is that it can be provided with more contained dimensions with respect to the burners currently available, so as to be used in atmospheric boilers without significantly modifying their layout.

Further characteristics and advantages of the present invention will become clear from the following detailed description of an embodiment of the invention in question, illustrated by way of non-limiting example in the attached figures wherein:

- Figure 1 illustrates a first embodiment of the burner according to the present invention;

- Figure 2 shows a sectional view of the burner of Figure 1 , on a median plane (M) which contains the longitudinal axis (X) of the burner;

- Figure 3 shows a schematic plan view of a battery of burners according to Figure 1 ; "AS" indicates schematically the plan area occupied by a heat exchanger associated with the battery of burners;

- Figure 4 illustrates a second embodiment of the burner according to the present invention;

- Figure 5 shows a sectional view of the burner of Figure 4, on a median plane (M) which contains the longitudinal axis (X) of the burner.

The burner according to the present invention comprises a Venturi conduit

(2) equipped with an inlet end (21 ), an outlet end (22) and a flue section (23).

The fuel is injected into the Venturi conduit through the inlet end (21 ) by means of a nozzle (9). As already mentioned, the flow of fuel determines the drawing of a determined flow rate of primary air through the inlet end (21 ).

The air drawn is partially mixed with the fuel along the Venturi conduit (2). The burner further comprises a mixing chamber (3) associated, at a first end, with the outlet end (22) of the Venturi conduit (2) and in which the mixing between the fuel gas and the primary air is completed. A diffusor (4) is associated with a second end of the mixing chamber (3). The Venturi conduit (2), the mixing chamber (3) and the diffusor (4) are aligned along a longitudinal axis (X).

An innovative and advantageous characteristic of the present invention with respect to the prior art is the nature of the diffusor. In the prior art, the diffusor comprises a sheet metal strip, or an appropriately perforated metal sheet from which the mixture of primary combustion air and fuel gas exits and to which the flame is anchored. Due to their nature, traditional low NOx modular burners allow poor or no power modulation since, due to the low burned power to burning surface ratios, the sheet metal strip overheats hence increasing the risk of flashback inside the mixing chamber as well as the risk of deformation and/or breakage of the burner due to thermal stress.

In the present invention the sheet metal strip is replaced by a pierced or porous metal or ceramic material. Materials of this nature are for example metal or ceramic mesh, metal or ceramic fabrics, metal or ceramic sintered materials, etc. A diffusor made with such materials has a much lower thermal conductivity than a sheet metal one. The burning surface can therefore reach high temperatures and ensure good anchoring of the flame without however transmitting heat onto the side facing the mixing chamber, hence preventing the aforementioned problems.

Thanks to the characteristics of the strip, the burner according to the present invention is able to withstand much higher power modulations than traditional burners of the same type. In particular, it can operate in radiant condition without the risk of flashback. This condition is an excellent compromise for maximising the efficiency of the burner and minimising pollutant emissions.

The other great advantage of these perforated or porous materials with respect to sheet metal is their mechanical resistance to thermal deformations. Sheet metal, if subjected to repeated thermal cycles at high temperatures, due to heat dilations, can be permanently deformed and even break. On the other hand, perforated or porous materials such as metal meshes or sintered ceramics do not undergo permanent deformations following repeated thermal cycles, even if they are exposed to high temperatures.

The diffusor (4) therefore comprises a body equipped with a plurality of through openings through which the mixture of primary air and fuel flows outside the burner in order to supply the combustion. Preferably the diffusor

(4) comprises a net support element to which a metal and/or ceramic mesh element is associated. The structure of the diffusor (4) determines, in a flow rate of gaseous fluid (Q) such as the mixture of primary air and fuel, a load loss (DP) that can be measured in Pascals. The diffusor (4) is preferably structured so that it determines, for an air flow rate (Q) at the temperature of 25 ° C measured in n /s that flows across a square surface of 78 cm ² , a load loss (DP) measured in Pascals equal to:

(a) DP = aQ ² + bQ

with 60300 < a < 79000 [kg*m ^"7 ] e 2500 < b < 2600 [kg*sec ^"1 m ^"4 ].

The load loss described by relationship (a) is lower with respect to the load losses generated by the typical sheet metal diffusors of the prior art. A diffusor (4) that produces a load loss in accordance with condition (a) allows optimal draught of the Venturi conduit (2) promoting the drawing of primary combustion air. This fact, along with the thermal and mechanical properties of the materials used in the present invention and described previously, enables the dimensions of the Venturi conduit (2) associated with the described diffusor (4) to be reduced, and to prevent the flashback phenomenon and inhibit the deformations/breakages associated with thermal stress. Furthermore, the increase in the quantity of primary air drawn substantially reduces the NO _x emissions.

The applicant has also noted that the use of the following relationships for designing the burner enables the efficiency and emission performance levels required by the market to be reached, maintaining an excellent level of compactness.

Considering that the diffusor (4) has a combustion surface having a complex area (A ₄ ), the ratio between it and the area (A23) of the flue section

(23) of the Venturi conduit (2) must be comprised between 20 and 43, i.e. :

(fa) 20 <— - < 43.

A third condition to be respected regards a relationship between the nominal power burned by the burner (P) and the total height (H-i) of the burner. The nominal power burned (P) by the burner is defined by the product between the flow rate of fuel gas that is injected by the nozzle (9) and the lower calorific power of the gas injected. The ratio between the nominal power burned by the burner (P) and the total height (H-i) of the burner must be comprised between 6 and 18 W/mm, i.e. :

p

(c) 6 <— < 18 fW/mml.

The height (H ₃ ) of the mixing chamber (3) is preferably comprised between 10 and 50 mm.

(d) 10 < H ₃ < 50 [mm]

A fifth condition to be respected relates the volume (V ₃ ) of the mixing chamber (3) to the total height (H-i) of the burner, meaning the distance between the inlet end (21 ) of the Venturi conduit (2) and the second end of the mixing chamber (3). The ratio between the volume of the mixing chamber (V ₃ ), measured in cm ³ , and the total height (H-i) of the burner, measured in cm, is comprised between 3 and 5, i.e. :

"(e) " 3 < ≤ 5 " lantfZ].

The design of a burner that respects the five above-listed conditions enables the dimensions of the burner itself to be substantially reduced, and an optimal quantity of primary air to be drawn. As already mentioned, this determines a consistent reduction in the quantity of nitrogen oxides produced during combustion.

A further condition that allows the production of NO _x to be reduced envisages sizing the area (A ₄ ) of the combustion surface of the diffusor (4) according to the nominal power burned (P) by the burner, so that the ratio between the power burned by the burner (P) and the area of the combustion surface is comprised between 70 and 150 W/cm ² :

(f) 70 < - - < l5o . _2- [W ^* cm ^d ]

From a geometric point of view, the diffusor (4) has two preferred conformations.

In a first embodiment the diffusor has an axysym metrical conformation with respect to the longitudinal axis (X) of the burner. The projection of the diffusor (4) on a perpendicular plan to the longitudinal axis (X) is substantially circular. In this embodiment the mixing chamber (3) is cylindrically or conically shaped.

In a second embodiment the diffusor (4) has an elongated conformation, symmetrical with respect to a median plane (M) on which the longitudinal axis (X) lies. The projection of the diffusor (4) onto a perpendicular plane to the longitudinal axis (X) has two straight and parallel lines, connected at the ends by two semi-circumferences with concavity facing inwards. In this second embodiment the mixing chamber (3) preferably, but not exclusively, has a diverging conformation from the outlet end (22) of the Venturi conduit (2) to the diffusor (4).

In both cases the width (L ₄ ) of the diffusor (4) is preferably comprised between 8 and 50 mm.

(g) 8 < L ₄ < 50 mm

In the first embodiment the width of the diffusor is substantially the diameter of the projection of the diffusor (4) on a perpendicular plane to the longitudinal axis (X) of the burner. In the second embodiment the width (L ₄ ) of the diffusor is substantially the distance between the two straight and parallel lines of the projection of the diffusor (4) onto a perpendicular plane to the longitudinal axis (X).

Preferably the diffusor (4) has a curved conformation, with a concavity facing towards the mixing chamber (3), whose minimum radius of curvature is constant in a neighbourhood of any point within its surface. In other words, in the first embodiment the diffusor (4) is in the form of a spherical cap. In the second embodiment the diffusor (4) is in the form of a cylindrical surface portion connected to the ends by two spherical surface portions.

A determined number of burners (1 ) according to the present invention may be associated with each other to form a modular burner of the type shown in Figure 3 in a top view.

In a preferred embodiment of the modular burner, the burners (1 ) are alongside each other according to a convenient spatial distribution. The modular burner is associated with a heat exchanger, of which only the plan outline is schematically shown, in which a fluid to be heated flows.

(A _b ) is the summation of the projections on a perpendicular plane to the longitudinal axis (X) of the surfaces of the diffusors (A ₄ ) of the burners (1 ) that comprise the modular system (Figure 3a).

(A _s ) is the projection on the same perpendicular plane to the longitudinal axis (X) of the surface of the heat exchanger associated with the modular burner.

The surface (A _b ) is contained in (A _s ).

Preferably between the surfaces (A _b ) and (A _s ) the following relationship is valid:

(h) 0.5 < A _b / A _s < 1 .

In other words the projection of the burning surface on a perpendicular plane to the longitudinal axis (X) must occupy at least half of the projection of the surface of the exchanger on the same plane.